In general, an SDH ring (it should be noted, in any case, that the field of application of the invention is not limited to this specific configuration) is made up of a set of synchronous devices capable of carrying out add-drop functions of low bit-rate signals into and from high bit-rate streams. Each node is connected to the two adjacent nodes through one or more unidirectional connections so as to form a closed path. The ring architecture allows providing protection against line and device failures and against degradation in transmission performance. In the ring, a part of the transmission capacity is dedicated to protection and therefore is not normally utilized to convey traffic. The protection capacity may possibly be used to transport very low priority traffic, which may be interrupted when a protection intervention on the ring is needed.
In ring topologies studied and provided until now, protection is accomplished by operating electrical cross-connections at the multiplex section or the path level, as described in ITU-T Recommendation G.803.
For example, in two-fiber bi-directional rings, traffic in one direction is to travel on one fiber while traffic in the opposite direction is to travel on the other fiber. Protection is achieved at the multiplex section level. These rings are also called "multiplex section shared protection rings" because, when a protection intervention is carried out in them, the transmission capacity reserved for protection is shared by the various channels to be protected.
In a two-fiber ring of this type, half the capacity of each fiber is dedicated to working traffic and the other half to protection traffic. If, for instance, each fiber conveys a 622 Mbit/s stream (defined as STM-4 in ITU-T Recommendations G708, G.709) deriving from the overlapping of four so-called administrative units (AU-4), two of these units are allotted to working traffic and two to protection traffic.
In regular operation conditions, the hi-directional communication between the two nodes utilizes only the "working half" of the fiber capacity: one administrative unit for one direction and one for the other. In case of failure, the two nodes adjacent to the point where the failure has occurred re-route the traffic of the working part of each fiber on the protection part of the other fiber, where transmission occurs in the opposite direction. The remaining nodes of the ring do not carry out any re-routing and continue to operate as they did before the failure onset.
The ring reconfiguration, when performed electrically, implies an inefficient exploitation of the available resources since half of the administrative units are intended for protection.
There is, then, a perceived requirement to have the possibility of carrying out a reconfiguration intervention on the ring structure optically, so as to allow full exploitation of the transmission capacity.